Characterisation and Modeling of Coupled-Core Optical Fibers

Doktorsavhandling, 2024

The growth in data traffic coupled with trends in internet use will result in a requirement for interfaces of the network to reach Tb/s data rate in the future. Considering this, novel transmission techniques that can increase the data rate with orders of magnitude must be considered. Coupled-core fibers (CCFs) have several closely spaced cores in the same cladding, which allows carry more data in the spatial domain with slower accumulation of group delay spread (GDS) and higher tolerance to nonlinearities. Application of CCFs in communication systems, though, is coupled with distortions of the signal due to GDS and other effects. They can be calculated, studied and partially mitigated if the transfer function of the fiber under test is known. Thus, it is essential to characterise the fiber's transfer matrix using fast and accurate measurement techniques. These characterisation measurements can also be used for building channel models that assist in simulations of the transmission and estimation of ultimate system performance.

In this thesis a novel method for fiber's characterisation based on dual-comb spectroscopy (DCS) and swept-wavelength interferometry (SWI) is proposed and evaluated. DC-SWI is studied in terms of capabilities, advantages and limitations with application on a CCF with three cores, for which the transfer function was measured. It is found that DC-SWI enables measurement of the broadband features that can not be measured using DCS and provides flexible trade-offs on SNR and frequency resolution. Unlike in SWI, in this experimental scheme it is not necessary to construct an additional interferometer for laser's sweep nonlinearity compensation. Furthermore, this thesis discusses proposed random coupling models that describe the linear properties of CCFs. The application of these models is investigated for CCFs with three and four cores. Modeled results show very good agreement with theory and measured data, which paves the way for using these models in DSP tests, simulations and investigation of installed fibers.